In the case where we have a co-dependent forward port (co-dependent
PRs got forward-ported, which they should be together) where *one* of
the PRs got explicitly updated, the batch would "fall into a hole"
being handled as neither "this is part of a forward-port sequence" nor
"this is a new merge to forward-port" (the latter being the proper
one).
Modify & remove guards which checked that either no or all PRs in a
batch have parents: should be either all or not all.
Fixes#231
